Hi Richard,
Thank you for reaching out.
To help you clear things out, production accounts and test accounts cannot be associated as these 2 have different purposes. Production accounts are used for serving ads and can generate cost and metrics, while test accounts are mainly used for testing your ads. These 2 types have very different environments.
If you wish to create test client accounts, you would need to have a test manager account first. You can create one by following this
guide. Your developers will use the OAuth2 credentials generated by the login email of your test manager account. You should also be able to use your current developerToken from your production MCC account to make calls using the API against your test accounts. If your developer token is still unapproved (test account access level) then it should be okay as this should be sufficient to run this. This
guide will cover most of your concerns regarding test accounts.
As for production environment, your developers should use the OAuth2 Credentials generated by the login email of your production manager account and use an approved (with basic access level) developer token to make your requests in the production environment.
Regards,
Dannison Heinrich Yao
Google Ads API Team

ref:_00D1U1174p._5001UEHF1l:ref